Head-up displays and sights available on the market today present a large size and an expensive price tag. This makes them unsuitable for small aeroplanes and helicopters. Most displays have no particular method for handling with different contrast between superimposed symbols and the outside view.
U.S. Pat. No. 5,596,451 disclose an assembly for producing a visual display including a light-reflecting type spatial light modulator and illumination arrangement for providing a particular source of light.
U.S. Pat. No. 7,015,876 disclose a head-up display for a vehicle which is in contrast to the windshield behind the display. In one embodiment the windshield is tinted to be in contrast with the colour of the head-up display. In other embodiments, the image forward of the vehicle is captured and sent to a control for the head-up display. The head-up display is then modified to be in contrast with the environment approaching the vehicle.
JP 2227340 discloses means for changing a display method according to a state of forward sight observed by a forward sight observation means, in a head-up display. A signal processing part processes information being inputted out of the outside, and determines type, colour, brightness or a display position of a symbol to be displayed to an operator, thereby outputting a signal requisite for the display toward a cathode-ray tube. In addition, forward sight photographed by a colour camera is subjected to picture processing, and when a colour of a symbol to be displayed is the same as the forward sight, the signal processing part changes the display position to a position where contrast is easy to recognize.
US 2006,103,590 disclose a mobile display system for a vehicle wherein a light source changes its intensity according to said vehicles driver's field of view light conditions.
KR 2004,019,453 disclose a device for adjusting colour and brightness of a head-up display comprising a charge coupled device (CCD) camera for monitoring display figures and a control unit for computing colour and brightness of the head-up display by the change of a front scene monitored by the CCD camera and for generating and outputting a control signal.
US. 2007,018,887 disclose a display system shoving semi-transparent polygons blended with the display's background terrain colour, but these polygons can change colour and be highlighted to match standard awareness and warnings as threat status is changed.
US 2006,024,1821 disclose a processor configured to control an LCD projection unit during unusual attitude of an aircraft to produce an attitude indicator of the aircraft and a horizon symbol. The horizon symbol divides the attitude indicator into a first region having a first display characteristic and a second region having a second display characteristic different from the first display characteristic.
US 2001,008,394 disclose a head-up display having a light source which intensity is adjusted to match a detected intensity of ambient light and the luminance of the light source is controlled depending on the ambient intensity and the light-controlled illumination.
U.S. Pat. No. 4,977,401 discloses a head-up display for indicating the attitude and flight path angle vertically, the attitude in roll and preferably the heading attitude and heading track angle of a vehicle. The indicated information is shown as if the aircraft with the display is located in the centre of an imaginary, space stable sphere with latitude circles and meridian markings.
There is a need for low-price, non-bulky head-up displays suitable for use in small vehicles such as small aeroplanes and helicopters, and which has a system for providing good contrast between generated symbols and the background terrain colour.
One object of the present invention is therefore to provide a head-up display system which include means for providing good contrast between generated symbols and the background terrain intensity and colour, and preferably not adding substantial costs to the head-up display system.